Convert struct object to object_id

struct object is one of the major data structures dealing with object
IDs.  Convert it to use struct object_id instead of an unsigned char
array.  Convert get_object_hash to refer to the new member as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>
diff --git a/merge.c b/merge.c
index fcff632..5db7d56 100644
--- a/merge.c
+++ b/merge.c
@@ -10,7 +10,7 @@
 static const char *merge_argument(struct commit *commit)
 {
 	if (commit)
-		return sha1_to_hex(commit->object.sha1);
+		return oid_to_hex(&commit->object.oid);
 	else
 		return EMPTY_TREE_SHA1_HEX;
 }